This invention relates to refrigeration systems of the overfeed type and particularly relates to ammonia refrigeration systems in low-temperature applications.
Especially, it relates to a refrigeration system having an evaporator overfed with liquid refrigerant and discharging a mixture of vapor refrigerant and liquid refrigerant; a compressor for compressing vapor refrigerant discharged from the evaporator; a condenser receiving compressed vapor refrigerant from the compressor for transforming it into liquid refrigerant; and a receiver receiving the liquid refrigerant from the condenser and supplying it to the evaporator.
Industrial refrigeration systems, particularly low-temperature, e.g. below -35.degree. F., high-capacity, e.g. greater than 25 TR, systems, are often overfeed type systems. In order to maximize the effectiveness of the evaporator in such refrigeration systems, its entire inner surface should be covered with liquid refrigerant, i.e. wetted. To wet the entire surface, excess liquid refrigerant of at least three times, preferably four times, that which is evaporated, must be fed into the evaporator. In addition, the refrigerant liquid fed into the evaporator should be at the sane temperature as the evaporator.
In typical refrigeration systems, the liquid refrigerant is flashed to evaporating temperature in a large vessel. After dropping to evaporating temperature, the refrigerant liquid is driven into the evaporator by one of several means. Mechanical pumps are used when the vessel is located remote from the evaporator. Mounting the vessel near the evaporator and above it allows gravity head to pressurize the cold liquid refrigerant and drive it through the evaporator.
Both of the above methods of driving the refrigerant through the evaporator require a large storage vessel to provide a pressure head for the mechanical pump or to provide a certain gravity head. Also, in each of these designs, there is a vertical lift in the pipe leaving the evaporator and/or horizontal pipe runs with two phase flow. The pressure drop is much greater in pipes with two phase flow than in those with a flow of only dry vapor. Excessive pressure drop results in higher operating costs and the need for larger compressors, mains, and vessels, raising the initial costs of the system. The large vessels required for these types of systems also require that the system be charged with a large amount of refrigerant.